High density barrier film separators



HIGH DENSITY BARRIER FILM SEPARATORS Fred L. Granger, Cleveland, Ohio, assignor to Union Carbide Corporation, a corporation of New York No Drawing. Application April 18, 1955 Serial No. 502,187

Claims. (Cl. 136-146) The instant invention relates to separators for use in primary galvanic cells, and to their methods of production.

Separators of this type frequently consist of a paper carrier, the anode side of which is coated with a highly bibulous material hereafter designated as an anode contacting film. in addition to serving as a carrier for the said film, the paper also prevents its dissipation and migration, and therefore may be called a barrier film.

In separators for conventional primary galvanic cells, a chief requisite is high electrolyte absorbency, so as to permit the retention of a maximum amount of electrolyte at the anode regions. In line with this requirement, only highly absorbent low density papers such as that manufactured from a loose mat of kraft fibers and the like have been considered and employed for this purpose.

It has been determined, however, that although a large amount of electrolyte at the anode is desirable, the bibulous coating alone fulfills that requirement, and, therefore, there is no necessity for the carrier paper itself to be a highly absorbent, low density sheet.

It is, therefore, an object of this invention to provide a paper separator and methods of production thereof, which separator will have optimum ion permeability and high stability to cell environment, among other desirable properties. Another object is to provide a dry cell separator consisting of unregenerated cellulose and other fibers of both the alkali and water-soluble varieties of cellulose ethers. A further object is to provide improved performance of dry cells owing to the incorporation therein of the barrier films of the invention.

In accordance with the present invention, improvements in dry cell behavior have been obtained with relatively non-absorbent high density, hard surfaced glazed papers having several technical characteristics:

(1) Thinness, to provide a minimum ionic path be tween the anode and cathode;

(2) Low internal resistance against ionic movement;

(3) smoothness, to prevent mixing of the bibulous anode contacting film with the carrier, which could impede ionic movement; and

(4) Tightness, to provide maximum effectivenes as a barrier, preventing the anode contacting film from being imbibed into the carrier and away from the anode. Theseproperties are provided in accordance with the method of the invention in a mechanically densified sheet resembling in properties and appearance, glassine, double glazed, flint glazed and friction glazed sheets.

Starting raw materials suitable for use are purified cellulose fibers such as kraft, sulfite, alpha cellulose, rag fiber and the like. In a variant of the present invention, up to 40 percent by weight of a chemically hydratable substituted cellulosic fiber is added.

The paper barrier or carrier sheets of the present invention markedly contribute to the service and shelf life of dry cells in which they are incorporated. They also possess high stability in conventional dry cell electrolytes. Their ability to absorb such electrolytes is lower than that 1 atet il a'tenteci Feb. 3, 1%5'9 of present type paper separators, but the thickness at which the required mechanical strength for this application can be obtained is much reduced, with a consequent reduction in ionic path, and a resulting increase in ionic permeability.

Two procedures for imparting the desired degree of hydration to the furnish or charge which is blended in the paper heaters are available. One of these is mechanical, and consists in partially defibrillating the furnish by employing beating as normally practiced in manufacture of glassine stock. The other is chemical, and consists in incorporating with the furnish a portion of chemically hydratable electrolyte-soluble cellulosic material. Wherever convenient, a combination of both procedures may, of course, be used.

In the first-mentioned approach, use is made of a fiber stock which has not been thoroughly hydrated by the heating process. Use of such material results in the obtention, after subjecting the same to super-calendering or glazing, of a density considerably less than that of pure cellulose. Where a material obtained as hereinabove described was used in a size F dry cell containing an anode, an insoluble cathode, a depolarizing mix, electrolyte and an anode contacting material, the following service data indicates the degree of improvement thus obtained as compared with a highly absorptive carrier during an 8 ohm continuous discharge test to 0.75 volt.

In the second procedure for obtaining the desired barrier characteristics previously indicated, an additional ingredient such as a cellulose ether is incorporated in a conventional glassine-type furnish. Examples of such additives are electrolyte semi-soluble fibrous materials such as methyl cellulose, hydroxy ethyl cellulose, or carboxy methyl cellulose. In this connection, it is well to mention that the extent of permeability, bibulousness and solubility of a given cellulosic ether depends to an extent on the degree of substitution. In the case of methyl cellulose, a low methoxy substitution results in a less bibulous and less permeable material. Generally a low substitution ether should be used in order that its water solubility would not be such as to cause loss of material in the paper-making process itself.. However, again in the case of methyl cellulose where the extent of substitution is less than 12 percent methoxyl radical by weight, the film is not sufliciently bibulous or permeable at low temperature to permit satisfactory cell operation. For general application, the methoxy weight percentage on the basis of the total methyl cellulose weight should range between 12 percent to 29 percent, with the optimum range extending from 18 percent to 22 percent. The overall solid composition of a furnish for use in the preparation of the separators of the invention should consist of about 60 percent to percent of mechanically hydrated cellulose to 40 percent to 0 percent of the chemically hydratable cellulose ether with the optimum cellulose ether content being about 20 percent by weight of the total charge.

In connection with the higher substitution range above mentioned, it is known that the wet strength of such fibers is such that they may show a tendency to split or break when wet, but this condition may easily be remedied by supporting the fibrous material on a roll or .5 forarninous support and the addition of well known Wet strength agents.

It is, of course, obvious that the thickness of the barrier films produced by the practice of this invention may be varied over wide dimension ranges, depending upon the characteristics of the cells in which they are to be employed. The internal resistance of a cell, however, is increased by a thickening of the barrier sheet, so that normally thicker sheets should be employed only where prolonged storage conditions and light cell drains are anticipated. It has been found that the material of the invention possesses great physical strength, even when in extremely thin sections of about 0.02 millimeter. For most applications a satisfactory dimension range is one between 0.02 millimeter and 0.12 millimeter.

The porosity of the paper produced by the method of this invention is of paramount importance, as sufficient pore space must be allowed for ion diffusion to permit cell reaction. Conventional measurements of this property by, for example, zinc ion movement rates through the paper by diffusion driving forces as against electrophoretic driving forces give conflicting results. Accordingly the measure of this property is defined in terms of air resistance as determined by the test of ASTMD 726-48. A satisfactory air resistance range as determined by that test is to 1000 seconds.

For more indicative results of performance to be expected in the dry cell environment, it has been found useful to pretreat the sample specimen to be tested for 48 hours in a typical dry cell electrolyte composition (23% Zn-Cl 28% NH Cl; pH 4.42; sp. gr. 1.285 at C.) wash the same With water and dry. After this treatment, the most effective range of air resistance values is 1 to 100 seconds.

Another property which has importance in the securing of controlled ion permeability is density. In this connection, it was found that a density range of 0.6 to 1.4 is satisfactory for most applications. This is in direct opposition to present battery technology where the absorbent type paper carriers have densities of 0.3 to 0.6.

As an example of the practice of the instant invention, a typical kraft process pulp was suspended in Water, and subjected to the action of a Jordan engine, until the fibers were partially defibrillated, or until the fibrils became partially loosened from the starting fiber, and appeared broomed under microscopic examination. The fibrous slurry resulting from the above process was then passed to a suitable paper machine of the foraminous support type, such as, for example, a Fourdrinier machine, where it was shaped into a sheet of the required thickness. The thus prepared paper sheet was then passed over a drying cylinder and subjected to conventional super-calendering process. After applying to this sheet an anode contacting soluble film such as that described in U. S. Patent 2,534,336 it was assembled in a zinc anode dry cell, and the cell showed excellent shelf and service life.

As another example of the invention, 20 percent by weight of methyl cellulose having a 20 percent methoxy content was added to a charge consisting of a typical glassine type furnish and blended in a suitable paper beating machine. The resulting fibrous slurry was then passed to a suitable paper machine of the foraminous support type, such as a Fourdrinier machine, where it was shaped into a sheet of the required thickness. This sheet was dried and super calendered and barrier films of suit- Minutes Absorbent alpha cellulose film carrier 793 Glassine type sheet containing 20% of cellulose ether as film carrier 870 From the foregoing it will be apparent that the instant invention provides new supporting means for conventional anode contacting films, and improves upon prior art separators which have been technically and economirally unsound. The separators so produced contribute extensively to the shelf and service life of dry cells in which they are assembled.

What is claimed is:

1. A dry cell separator consisting of an anode-contacting film and a relatively smooth surfaced, ionically permeable, high density, super-calendered sheet of paper, formed from partially mechanically defibrillated cellulosic fibers and up to 40 percent by weight thereof of alkali soluble, partially water soluble, cellulosic ethers.

2. A dry cell separator consisting of an anode-contacting film and a relatively smooth surfaced, ionically permeable, high density, super-calendered sheet of paper formed from chemically purified cellulosic fibers, and up to 40 percent by weight thereof of methyl cellulose ether having about 12 percent to 29 percent methoxy content based on the dry weight of said methyl cellulose ether.

3. In a dry cell comprising an anode, an insoluble cathode, a depolarizer mix, an electrolyte, a fibrous, ionically permeable barrier film supporting an anode contacting material abutting said anode, intermediate said depolarizer mix, said barrier film comprising 20 percent of methyl cellulose ether having an 18 percent methoxy content based on the dry weight of the methyl cellulose ether and percent of mechanically hydrated cellulose.

4. A dry cell separator consisting of an anode-contacting film and a relatively smooth surfaced, ionically permeable, low absorption, dense, super-calendered sheet of paper, said paper having an air resistance in the range of 5 to 1000 seconds (Gurley) and a density of 0.6 to 1.4 gm./cc.

5. In a dry cell comprising an anode, an insoluble cathode, a depolarizer mix, an electrolyte, a fibrous ionically permeable barrier film supporting an anode-contacting material abutting said anode, intermediate said depolarizer mix, said barrier film consisting of a high density, super-calendered sheet of paper having a porosity within the range of 5 to 1000 seconds (Gurley) and a density in the range of 0.6 to 1.4 gm./ cc.

References Cited in the file of this patent UNITED STATES PATENTS 1,808,410 Heise et al. June 2, 1931 1,890,178 Heise et al. Dec. 6, 1932 2,273,677 Nallach Feb. 17, 1942 2,292,118 Guhl Aug. 4, 1942 2,534,336 Cahoon Dec. 19, 1950 2,543,137 Uber Feb. 17, 1951 

1. A DRY CELL SEPARATOR CONSISTING OF AN ANODE-CONTACTING FILM AND A RELATIVELY SMOOTH SURFACED, IONICALLY PER MEABLE, HIGH DENSITY, SUPER-CALENDERED SHEET OF PAPER, FORMED FROM PARTIALLY MECHANICALLY DEFIBRILLATED CELLULOSIC FIBERS AND UP TO 40 PERCENT BY WEIGHT THEREOF OF ALKALI SOLUBLE, PARTIALLY WATER SOLUBLE, CELLULOSIC ETHERS. 